1. Field of the Invention The present invention relates to a display device, and more particularly, an organic electroluminescent display device and a method of fabricating the same.
2. Discussion of the Related Art
Generally, an organic electroluminescent display (OELD) device emits light by injecting electrons from a cathode and holes from an anode into an emission layer, combining the electrons with the holes, generating excitons, and transitioning the excitons from an excited state to a ground state. In contrast to a liquid crystal display (LCD) device, an OELD does not require an additional light source because the OELD device transmits light emitted by the transition of the excitons between states. Accordingly, the OELD device is lighter and smaller than a comparable liquid crystal display (LCD) device. The OELD device has other desirable characteristics, such as low power consumption, superior brightness and a fast response time. Because of these advantageous characteristics, the OELD device is regarded as a promising candidate for use in various next-generation consumer electronic applications, such as cellular phones, car navigation systems (CNS), personal digital assistants (PDA), camcorders and palmtop computers. Moreover, an OELD device is much cheaper to produce than an LCD device because the fabrication process is relatively simpler for the OELD device than the LCD device and has fewer processing steps. There are two different types of OELD devices: passive matrix and active matrix.
FIG. 1 is a cross-sectional view of an organic electroluminescent device according to a related art. Referring to FIG. 1, an OELD device 10 includes a first substrate 12 and a second substrate 28 facing and spaced apart from each other. An array element layer 14 is formed on an inner surface of the first substrate 12. The array element layer 14 includes including a thin film transistor (TFT) T. A first electrode 46, an organic electroluminescent (EL) layer 50, and a second electrode 52 are sequentially formed over the array element layer 14. The organic EL layer 50 may separately display red, green, and blue colors for each pixel region P.
The first substrate 12 and the second substrate 28 are attached with a sealant 26. The OELD device 10 is encapsulated by attaching the first substrate 12 to the second substrate 28. A moisture absorbent desiccant 22 is positioned on the second substrate 28. The moisture absorbent desiccant 22 eliminates moisture and oxygen that may penetrate the encapsulated organic EL layer 50. More particularly, a portion of the second substrate 28 is etched and the moisture absorbent desiccant 22 is placed in the etched portion and affixed by a holding element 25. Although not shown, the organic EL layer may be divided into a plurality of pixel regions by a separator.
FIG. 2 is a plan view of a separator in an organic electroluminescent device according to the related art. Referring to FIG. 2, a first electrode 46 and an organic EL layer 50 (shown in FIG. 1) are located in the pixel region P. A second electrode 52 (shown in FIG. 1) is formed on an entire surface of the first substrate 12. For example, when the first electrode 46 acts as an anode, it is formed by depositing and patterning a conductive material having a high work-function, such as indium tin oxide (ITO), through a vacuum apparatus, such as a sputtering chamber. In addition, when the organic EL layer 50 is made of a polymeric material, it is formed by a printing process, for example ink-jet printing. On the other hand, when the organic EL layer 50 is made of a monomeric material, it is formed by a deposition process.
A separator SP is required to divide the organic EL layer 50 into a plurality of pixel regions P in an independent emitting type OELD device using a polymeric organic EL material. The separator SP can prevent mixing between different colors of the organic EL layers 50. Although not shown, the buffer layer 48 and the separator SP correspond to portions of the gate, data and power lines.
The organic EL layer 50 includes red, green and blue EL layers (not shown) formed in the pixel regions P in repeating order after forming the separator SP at a boundary of the pixel regions P and a buffer layer 48 on the separator SP. The buffer layer 48 is located at the boundary of the pixel regions P including a portion overlapping edges of the first electrode 46 to prevent any electrical contact between the first electrode 46 and the second electrode 52 (shown in FIG. 1) at the corner of the separator SP. Thus, the buffer layer 48 is larger than the separator SP.
FIG. 3 is a plan view of an array substrate of an organic electroluminescent device for one pixel region according to the related art. In general, an array element layer 14 (shown in FIG. 1) of an OELD device 10 includes a switching thin film transistor TS, a driving thin film transistor TD and a storage capacitor CST. A first substrate 12 is made of a transparent insulating substrate, such as glass and plastic. A gate line GL and a data line DL cross each other are formed on the first substrate 12. The gate line GL and the data line DL define a pixel region. An insulating layer (not shown) is interposed between the gate line GL and the data line DL. A power line PL crosses the gate line GL, in parallel with and spaced apart from the data line DL.
The switching thin film transistor TS includes a switching gate electrode 26, a switching active layer 16, a switching source electrode 34, and a switching drain electrode 36. Similarly, the driving thin film transistor TD includes a driving gate electrode 28, a driving active layer 18, a driving source electrode 38, and a driving drain electrode 40. The switching gate electrode 26 is connected to the gate line GL and the switching source electrode 34 is connected to the data line DL. The switching drain electrode 36 is connected to the driving gate electrode 28 via a first contact hole 69 that exposes a portion of the driving gate electrode 28. The driving source electrode 38 is connected to the power line PL via a second contact hole 57 that exposes a portion of the power line PL. Moreover, a first electrode 46 is connected to the driving drain electrode 40 via the third contact hole 59. The power line PL overlaps a first capacitor electrode 20 with the insulating layer interposed therebetween to form the storage capacitor CST.
Although not shown, the separator SP (shown in FIG. 2), which is formed in a portion corresponding to the data line DL and the power line PL, can divide the organic EL layer emitting a specific light into a plurality of pixel regions. Further, the buffer layer 48 (shown in FIG. 2) is formed between the first electrode 46 and the separator SP and is located in the non-pixel region at a boundary of the pixel region P including the portion overlapping edges of the first electrode 46.
FIGS. 4A, 4B and 5 are cross-sectional views taken along lines IVA-IVA, IVB-IVB and V-V of FIG. 3, respectively. Referring to FIGS. 4A, 4B and 5, a switching thin film transistor TS and a driving thin film transistor TD are formed on a first substrate 12 including a switching region S and a driving region D within a pixel region P, respectively. The switching thin film transistor TS includes a switching active layer 16, a switching gate electrode 26, a switching source electrode 34, and a switching drain electrode 36. Similarly, the driving thin film transistor TD includes a driving active layer 18, a driving gate electrode 28, a driving source electrode 38, and a driving drain electrode 40.
Specifically, although not shown, the switching gate electrode 26 is connected to the gate line GL and the switching source electrode 34 is connected to the data line DL. The switching drain electrode 36 is connected to the driving gate electrode 28. The driving source electrode 38 is connected to the power line PL, and the driving drain electrode 40 is connected to a first electrode 46 in the pixel region P. A buffer layer 48 is formed on the first electrode 46 at a boundary of the pixel region P corresponding to the data line DL and the power line PL, as shown in FIG. 5. The buffer layer 48 overlaps edges of the first electrode 46. A separator SP is formed on the buffer layer 48 within the boundary of the pixel region P. An organic EL layer 50 is formed on the first electrode 46 in the pixel region P surrounded by the separator SP. A second electrode 52 is formed on the entire surface of the organic EL layer 50 and the separator SP. However, at least two mask processes are required to form the buffer layer 48 and the separator SP for the OELD device.
FIGS. 6A to 6E are cross-sectional views of a fabricating process of an organic electroluminescent diode substrate for an organic electroluminescent device according to the related art. Referring to FIG. 6A, a first electrode 46 is formed on a first substrate 12 including a pixel region P. The first electrode 46 is located in the pixel region P. An inorganic material layer 47 is formed by depositing an inorganic material, such as silicon nitride (SiNx), on the entire surface of the first electrode 46 and the first substrate 12. A photoresist layer 80 is formed by coating photoresist on the inorganic insulating material layer 47.
Referring to FIG. 6B, a photoresist pattern 82 is formed by patterning the photoresist layer 80 on the inorganic material layer 47 at the boundary of the pixel region P including a portion overlapping edges of the first electrode 46. Referring to FIG. 6C, a buffer layer 48 is formed by etching a portion of the inorganic material layer 47 (shown in FIG. 6B) uncovered by the photoresist pattern 82 (shown in FIG. 6B). Then, the buffer layer 48 is formed using a first mask process, which includes exposing, developing and etching. An organic layer 90 is formed by coating an organic insulating material on the entire surface of the buffer layer 48 and the first electrode 46.
Referring to FIG. 6D, a separator SP is formed by patterning the organic material layer 90 (shown in FIG. 6C) through a second mask process similar to the first mask process. The separator SP is located within the boundary of the pixel region P but the buffer layer 48 overlaps the edges of neighboring pixel regions P to prevent the first electrode 46 and a second electrode that will be formed later from electrically contacting each other. Thus, although not shown, a width of the buffer layer 48 is larger than a width of the separator SP. When an organic EL layer is formed by coating a polymeric material, the separator SP should have a height of more than 1 micrometer. Thus, a portion of the organic EL layer near to the separator SP becomes thicker. Accordingly, the separator SP should have a predetermined side gap K with the buffer layer 48 toward the first electrode 46.
Referring to FIG. 6E, an organic EL layer 50 is formed on the first electrode 46 in the pixel region P surrounded by the separator SP. A second electrode 52 is formed on the entire surface of the organic EL layer 50 and the separator SP. The second electrode 52 acts as a cathode and comprises a metallic material having a low work-function, such as calcium (Ca), aluminum (Al) and magnesium (Mg) and lithium fluorine/aluminum (LiF/Al). In addition, when the organic EL layer 50 is formed by coating, no mask process is required for forming the organic EL layer 50.
At least two mask processes are required to form the buffer layer 48 and the separator SP. As a result, several mask processes are required for the entire manufacturing process. The defective fraction increases with the number of mask processes. Moreover, production yield decreases and production cost increases because of processing delay, thus weakening a competitive pricing of the EL device.